The transformation of mixtures of methyleyelohexane and small amounts of phenol, added in growing quantities (0.01), 0.06, 0.6, and 1.2 wt%), was carried out over an H-ZSM-5 zeolite at 350 and 450 degrees C, in order to simulate the biomass-derived oils and fluid catalytic cracking feedstocks cofeeding. At 350 degrees C, with only 0.6 wt% of phenol fed with methyleyelohexane, a very fast and quasicomplete deactivation of the zeolite occurs. The main cause of deactivation is the strong adsorption of phenol molecules on the acid sites of the zeolite causing at the same time active-site and pore blockage. This could be due to the small size of the ZSM-5 pores that slows down the diffusion of phenol molecules and facilitates their adsorption and retention. This negative effect of phenol addition can be observed whatever the contact time, the reaction temperature, and even after only 2 min of reaction (first experiemental value). All this suggests that, depending on the amount of bio-oil incorporated in the classical FCC feedstock, remarkably different effects should be induced in the reduction of ZSM-5 additive action. Therefore, it would be neccessary to control the quantity of bio-oil that can be added to keep the ZSM-5 additive effect at an acceptable level.